Industrial plants which handle volatile organic compounds (VOCs) typically experience unwanted emissions of such compounds into the atmosphere from point sources, such as smoke stacks, and non-point sources, such as valves, pumps, and fittings installed in pipes and vessels containing the VOCs. Such VOCs include, but are not limited to, aromatics (e.g., benzene, toluene, ethylbenzene, and xylenes), halogenated hydrocarbons (e.g., carbon tetrachloride, 1,1,1-trichloroethane, and trichloroethylene), ketones (e.g., acetone, and methyl ethyl ketone), alcohols (e.g., methanol, ethanol, and propanol), ethers (e.g., dimethyl ether and methyl t-butyl ether), and aliphatic hydrocarbons (e.g., natural gas and gasoline).
Emissions from non-point sources, referred to as fugitive emissions, typically occur due to the leakage of the VOCs from joints and seals. Fugitive emissions from control valves can occur as the result of leakage through the packing between the valve stem and the body or bonnet of the valve. Valves employed in demanding service conditions involving frequent movement of the valve stem and large temperature fluctuations typically suffer accelerated deterioration of the valve stem packing, which results in greater fugitive emissions than valves employed in less demanding service.
While improvements in valve stem packing materials and designs have reduced fugitive emissions and lengthened the life of valve packing, the monitoring of fugitive emissions has become important as a means to identify and reduce fugitive emissions, and to comply with the more stringent regulation of emissions. For example, the Environmental Protection Agency (EPA) has promulgated regulations for specifying the maximum permitted emission of certain hazardous air pollutants from control valves, and requires periodic surveys of emissions from control valves.
Current methods of monitoring fugitive emissions involve manual procedures using a portable organic vapor analyzer. This manual method is time consuming and expensive to perform, and also can yield inaccurate results due to ineffective collection of the fugitive emissions from the equipment being monitored. If measurements are made on a valve exposed to wind, emissions from the valve may be dissipated before the analyzer can properly measure the concentration of the emissions. Also, if the analyzer is not carefully moved around the valve to capture all the emissions from the valve, an inaccurate measurement will result. Manual measurement methods also require plant personnel to dedicate a significant amount of time to making the measurements, thereby distracting plant personnel from other duties.
Automated monitoring and detection of fugitive emissions can yield significant advantages over existing manual methods. The EPA regulations require surveys of fugitive emissions at periodic intervals. The length of the survey interval can be monthly, quarterly, semi-annually, or annually, with the required surveys becoming less frequent if the facility operator can document a sufficiently low percentage of control valves exhibiting excessive leakage. Thus, achieving a low percentage of leaking valves reduces the number of surveys required per year. In a large industrial facility, where the total number of survey points can range from 50,000 to 200,000, a reduced number of surveys can result in large cost savings. By installing automated fugitive emission-sensing systems on valves subject to the most demanding service conditions, and thus, most likely to develop leaks, compliance with the EPA regulations can be more readily achieved for the entire facility.
However, employing chemical sensors in an industrial environment requires designing sensors that perform satisfactorily in the presence of high relative humidity across a broad temperature range. The sensors must be able to discriminate between the emissions of interest and other environmental contaminants, while retaining sufficient sensitivity to detect low concentrations of the fugitive emissions. A provision also must be made to enable periodic calibration of the sensors. The output signals from the fugitive emission sensing system must be suitable for input into plant monitoring and control systems typically found in process plants. This permits simple and inexpensive integration of the sensing system into existing plant process control systems.
The fugitive emission sensing system must be inexpensive to manufacture, and use a power source that is readily available in a typical process plant in order to keep installation costs to a minimum. The system must be suitable for use in hazardous areas subject to risk of explosion, requiring electrical equipment to be intrinsically safe or of an explosion-proof design. It also must be able to operate in harsh environments, including areas subject to spray washing, high humidity, high and low temperatures, vibration, oxidizing gases, and other gases which affect sensor performance. The system also must be simple and reliable, in order to keep maintenance costs to a minimum.
In certain applications, the sensors used to detect fugitive emissions are provided in the form of piezoelectric-based sensors having high sensitivities to mass changes, such that when an alternating potential is applied across the sensors, changes in resulting wave characteristics in the sensors, specifically the resonant frequency, indicate the presence of the analyte. More specifically, the sensors typically include a quartz crystal substrate with an outer layer made of material selected to most effectively absorb the analyte. Such outer coatings are selected to increase sensitivity, while reducing acoustic wave damping effects. In addition, such materials should be environmentally robust to accommodate the aforementioned wide temperature ranges, humidity ranges, and high levels of dust particles and other contaminants and oxidants.